Relax the muscles in your neck, mouth and face. If your muscles are too tense, you'll be less able to sing higher pitches well.
Stand up straight, take a deep breath and contract the muscles in your abdomen slightly as you begin to sing. Your upper abdominal muscles help to support your tone. Good posture enables you to use these muscles effectively and to breathe deeply. Having plenty of breath as you sing is also important when you are singing high notes.
Tuck in your chin and drop your jaw as you sing. This maximizes the vibration capacity in your mouth and facial "mask," which in turn, helps you to hit high notes.
Imagine that you are about to yawn. Notice how the rear part of the roof of your mouth lifts upward. This is called your soft palette. Consciously raise this as you sing falsetto notes.
Listen to music that features falsetto singing. The classic falsetto sound is characterized by the power of a man's voice with the highness of a woman's pitch. Try to imitate this as you sing.